Lithium batteries, and among these, lithium secondary batteries have characteristics such as a large energy density and a long life and have been used as a power supply of home electric appliances such as a video camera, and a portable electronic apparatus such as a notebook personal computer and a cellular phone. Recently, the lithium secondary battery has been applied to a large-sized battery that is mounted on the electric vehicle (EV), the hybrid electric vehicle (HEV), and the like.
The lithium secondary battery is a secondary battery having the following configuration. During charge, lithium is eluted as ions from a positive electrode, moves to a negative electrode, and is stored in the negative electrode. Conversely, during discharge, the lithium ions return to the positive electrode from the negative electrode. It is known that the high energy density of the lithium secondary battery is caused by an electric potential of a positive electrode material.
As a positive electrode active material of the lithium secondary battery, in addition to lithium-manganese oxide (LiMn2O4) having a spinel structure, lithium metal compound oxides such as LiCoO2, LiNiO2, and LiMnO2 which have a layered structure are known. For example, LiCoO2 has a layered structure in which a lithium atom layer and a cobalt atom layer are alternately overlapped with an oxygen atom layer interposed therebetween, and has large charge and discharge capacity and excellent diffusibility in storage and release of lithium ions. Accordingly, the majority of commercially available lithium secondary batteries are composed of a lithium metal compound oxide such as LiCoO2 having a layered structure.
The lithium metal compound oxides such as LiCoO2 and LiNiO2 which have a layered structure are expressed by general formula of LiMeO2 (Me represents a transition metal). A crystalline structure of the lithium metal compound oxide having the layered structure belongs to a space group R-3m (“-” is commonly attached to an upper portion of “3” and represents a rotary inversion; the same shall apply hereafter), and a Li ion, a Me ion, and an oxide ion occupy a 3a site, a 3b site, and a 6c site, respectively. In addition, it is known that the lithium metal compound oxide shows a layered structure in which a layer (Li layer) composed of Li ions and a layer (Me layer) composed of Me ions are alternately overlapped with an O layer composed of oxide ions interposed therebetween.
In the related art, with regard to the lithium metal compound oxide (LiMxO2) having a layered structure, for example, Patent Document 1 discloses an active material expressed by formula of LiNixMn1−xO2 (in formula, 0.7≦x≦0.95) which is obtained by adding an alkali solution to a mixed aqueous solution of manganese and nickel to coprecipitate manganese and nickel, by adding lithium hydroxide to the mixed aqueous solution, and by firing the resultant mixture.
Patent Document 2 discloses a positive electrode active material which is formed from crystal particles of oxides containing three kinds of transition metals, in which a crystal structure of the crystal particles is a layered structure and arrangement of oxygen atoms constituting the oxides is cubic closest packing, and which is expressed by Li[Lix(APBQCR)1−x]O2 (in formula, A, B, and C represent three kinds of transition metal elements different from each other, −0.1≦x≦0.3, 0.2≦P≦0.4, 0.2≦Q≦0.4, and 0.2≦R≦0.4).
Patent Document 3 discloses a method of manufacturing a layered lithium-nickel-manganese compound oxide powder to provide a layered lithium-nickel-manganese compound oxide powder having a high bulk density. The method includes drying slurry, which contains at least a lithium source compound, a nickel source compound, and a manganese source compound that are pulverized and mixed in a range of 0.7 to 9.0 in terms of a molar ratio [Ni/Mn] between a nickel atom [Ni] and a manganese atom [Mn] by spray drying, firing the resultant compound obtained by drying the slurry to form a layered lithium-nickel-manganese compound oxide powder, and pulverizing the compound oxide powder.
Patent Document 4 discloses a material which contains a lithium transition metal compound oxide obtained by mixing-in vanadium (V) and/or boron (B) to make a crystallite size large, that is, a lithium transition metal compound oxide expressed by general formula of LiXMYOZ-δ (in formula, M represents Co or Ni that is a transition metal element, and a relation of (X/Y)=0.98 to 1.02 and a relation of (δ/Z)≦0.03 are satisfied), and contains vanadium (V) and/or boron (B) in a ratio of ((V+B)/M)=0.001 to 0.05 (molar ratio) with respect to the transition metal element (M) that constitutes the lithium metal compound oxide. In the material, a primary particle size is 1 μm or more, a crystallite size is 450 Å or more, and a lattice strain is 0.05% or less.
An object of Patent Document 5 is to provide a positive electrode active material for a nonaqueous secondary battery which is formed from primary particles and which retains a high bulk density and battery characteristics without occurrence of cracking, and Patent Document 5 suggests a positive electrode active material for a nonaqueous secondary battery which is a powdered lithium compound oxide of monodispersed primary particles containing one kind of element selected from a group consisting of Co, Ni, and Mn, and lithium as a main component, and in which D50 is 3 μm to 12 μm, a specific surface area is 0.2 m2/g to 1.0 m2/g, a bulk density is 2.1 g/cm3 or more, and an inflection point of a volume reduction rate according to a Copper Plot method does not appear before 3 ton/cm2.
Patent Document 6 suggests a positive electrode active material for a nonaqueous electrolyte secondary battery. The positive electrode active material relates to a lithium metal compound oxide powder expressed by LizNi1−wMwO2 (provided that, M represents at least one or more kinds of metal elements selected from a group consisting of Co, Al, Mg, Mn, Ti, Fe, Cu, Zn, and Ga, and relations of 0<w≦0.25 and 1.0≦z≦1.1 are satisfied). The lithium metal compound oxide powder is composed of primary particles of the lithium metal compound oxide powder and secondary particles that are formed by agglomeration of a plurality of the primary particles. The shape of the secondary particles is spherical or oval spherical. 95% or more of the secondary particles have a particle size of 20 μm or less, an average particle size of the secondary particles is 7 μm to 13 μm, and a tap density of the powder is 2.2 g/cm3 or more. In pore distribution measurement according to a nitrogen absorption method, an average diameter is 40 nm or less, an average volume of pores is 0.001 cm3/g to 0.008 cm3/g. An average crushing strength of the secondary particles is 15 MPa to 100 MPa.
Patent Document 7 suggests a lithium metal compound oxide having a layered structure. In the lithium metal compound oxide, a ratio of a crystallite size to an average powder particle size (D50) is 0.05 to 0.20. The average powder particle size (D50) is obtained by a laser diffraction and scattering type particle size distribution measurement method after carrying out pulverization using, for example, a wet type pulverizer until D50 reaches 2 μm or less to obtain a crushed material, granulating and drying the crushed material using a thermal spray drier or the like, and firing the resultant granulated and dried material.